It sounded like a missile at first, the air above Washington filled with the terrifying roar of displaced air. Then the Pentagon was rocked by the thud of an explosion, and staff inside its fortified walls, who had been watching in horror the terrible images from New York, realised that the epicentre of US military might was also under attack.

The medium-sized jet had come in low over Arlington and the Navy Annexe, before screaming into the south-west face of the Pentagon around 9.30am.

"There was a huge noise and I got out of the car as the plane came over," said Afework Hagos, who was on his way to work but was stuck in a traffic jam near the Pentagon when the plane flew over.

"Everybody was running away in different directions. It was tilting its wings up and down like it was trying to balance. It hit some lampposts on the way in."

Omar Campo, who had been cutting the grass on the other side of the road when the plane flew over his head, said: "The whole ground shook and the whole area was full of fire. I could never imagine I would see anything like that here."

Barely 30 minutes after two other passenger jets had ploughed into the twin towers of the World Trade Centre in New York, the Washington attack tipped the US into a panic-fuelled state of siege. In that moment, what initially appeared to be a catastrophic but isolated terrorist outrage was transformed into an unprecedented full-frontal assault on America and its people.

From start to finish, the terrorist operation took barely three hours. In that time, those responsible managed to hijack four US airliners inside the supposedly well-guarded confines of US airspace and use them to reduce the country's two most important cities to war zone-like scenes of carnage.

By the time most east coast Americans had turned up at their desks, the operation was already well under way. Just after 8am, the terrorists had seized control of two airliners minutes after take-off from Logan airport in Boston. Another flight was hijacked shortly after leaving Washington Dulles, while the fourth had just left Newark, New Jersey.

But even after two of the jets had ploughed into the World Trade Centre less than an hour later, Americans still had no idea of the scale of devastation that was yet to unfold upon them. President George Bush was in Florida, visiting an elementary school where he had been reading stories with some of the pupils. As the scale of the carnage in New York became apparent, he cut the visit short and in a hastily convened news conference, announced that he was returning to Washington immediately.

But just as Mr Bush was appearing before the cameras, reports were emerging that another passenger plane had been hijacked. Military officials in Washington had been informed that the aircraft was heading in their direction from New York. Minutes later, the capital was thrown into chaos.

Tim Tinnerman, a pilot, watched as the airliner - which he said was an American Airlines Boeing 757 - hit the Pentagon. "It added power on its way in," he said. "The nose hit, and the wings came forward and it went up in a fireball."

"It was a huge fireball, a huge, orange fireball," said Paul Begala, a consultant with the Democratic party. Another wit ness also claimed the blast had blown up a helicopter circling overhead.

Inside the building, there was pandemonium. Terrified civilian and military staff were screaming as a serious fire took hold.

Smoke and flames poured out of a large hole punched into the side of the Pentagon. Emergency crews rushed fire engines to the scene and ambulancemen ran towards the flames holding wooden pallets to carry bodies out. A few of the lightly injured, bleeding and covered in dust, were recovering on the lawn outside, some in civilian clothes, some in uniform. A piece of twisted aircraft fuselage lay nearby. No one knew how many people had been killed.

Red, yellow and green sectors had been established on a nearby road, prepared to handle the different degrees of casualties once victims were brought out, but rescue workers were finding it nearly impossible to get to people trapped inside, beaten back by the flames and falling debris.

"The fire was intense," Rear Admiral Craig Quigley, the Pentagon spokesman, told reporters in a makeshift briefing at a gasoline station across the street from the building.

"It's terrible in there," said one firefighter, Derek Spector, who was with one of the first units to arrive at the scene. "But we didn't come across any casualties."

The regular Pentagon helicopter pad was not usable, scattered with debris from the plane and the explosion. But helicopters were landing and taking off from a cordoned-off area nearby. Within minutes, ambulances and a busload of trauma experts arrived from the army's Walter Reed hospital in Washington.

Law enforcement officials, speaking on condition of anonymity, said the plane that struck the Pentagon was the American Airlines jetliner that had taken off from Dulles on a scheduled flight to Los Angeles. Among the passengers was Barbara Olson, the wife of solicitor general Theodore Olson. Mrs Olson, a CNN commentator, had frantically called from her mobile phone to say her plane had been hijacked.

A spokesman for her husband later revealed she had not even been due to fly on the flight. "She flew a day early to make sure she could be at Ted's birthday," he said. "She called and said she was locked in the toilet and the plane had been hijacked. She said they had box-cutters and knives. They had rounded up the passengers at the back of the plane.

She referred to them as more than one. There was nothing she could do. She said to her husband: 'What do I do?'" The call ended seconds before the crash. Her husband, who had been George Bush's lawyer during the legal battle over the disputed presidential election, was said to be distraught.

The brunt of the impact had been taken by the third and fourth floors of the Pentagon's outer ring, which housed senior navy personnel, including three-star officers and vice admirals. There were also offices used by secretaries of the different armed services and the assistant secretaries. A Pentagon spokeswoman said the defence secretary, Donald Rumsfeld, escaped unharmed.

High alert Across the country, America began pulling up the drawbridges within minutes of the Pentagon attack. President Bush ordered US forces worldwide on to high alert status - force protection condition Delta - and the authorities immediately began deploying troops, including a regiment of light infantry, in Washington.

As the aviation authorities worked frantically to account for the safety of all airliners in domestic air space, every airport was closed down and all flights in US airspace were ordered to land. International flights en route to the US were diverted to Canada.

In Washington, all government buildings, including the state department, the Capitol building and the White House, had been evacuated after the New York attacks and the nine top leaders of the house and senate taken into federal protection. But as fears of further attacks spread, public buildings across the country were also evacuated as the government began shutting down national landmarks, including the Washington Monument, the Statue of Liberty and the St Louis Gateway Arch. Even Disneyworld in Orlando closed its doors.

On the streets of Washington, panic set in. People rushed from buildings and desperately tried to get to their children in schools and daycare centres to make sure they were safe. Drivers ran red lights and sped across intersections, sending pedestrians scattering in a bid to get out of the city. Police near the White House tried to direct traffic, but a few blocks away chaos reigned, thwarting the efforts of emergency vehicles.

"We are all sitting ducks here. We can't get out of the city. If they want to bomb the city we are all just waiting," one federal employee said.

"I feel like they are getting closer and closer with every minute," said Leroy Hall, a World Bank worker.

Just after 10am, the situation worsened again. Five minutes after the first World Trade Centre tower collapsed in New York, masonry started falling from the Pentagon. Then, without warning, a 40-yard section collapsed leaving a yawning gap from which flames continued to shoot. Stanley St Clair stumbled along the road away from the vast building, covered in dust. He had been working on renovations on the first floor of the section which was struck by the plane.

"It shook the whole building and hurt our ears. Papers and furniture and debris just went flying through the hallway and I thought it was a bomb or something. Then someone started shouting get out, get out."

Renovation work on the upper floors had just been completed and they had been handed back to the defence department. "This is the second Pearl Harbour. I don't think that I overstate it," Senator Chuck Hagel told reporters.

At 10.15am, another alert was sounded in Washington. "Get them out of here. We've got another threat coming," a policemen yelled, pushing survivors back from the building. Another officer said a report had come in saying another plane was on its way into Washington.

US air force F-16 fighter jets were scrambled, one of them banking steeply around the Pentagon, as the air around the defence department began to buzz with military and police helicopters.

At 10.27am in New York, the second tower of the World Trade Centre came tumbling down.

Minutes later, news broke of another crash, this time around 80 miles south-east of Pittsburgh, Pennsylvania. At 9.58am, an emergency dispatcher had answered a telephone call from a man who said he was a passenger locked in a bathroom on United Airlines flight 93. "We are being hijacked, we are being hijacked," he told the dispatcher, while repeatedly insisting that the call was not a hoax. The plane was "going down", he said. He had heard some sort of explosion and said there was white smoke coming from the aircraft. Then the dispatcher lost him.

The plane, a Boeing 757, which had left Newark, New Jersey, at 8.01am with 45 passengers and crew on board bound for San Francisco, had crashed into fields north of Somerset County airport. There were no survivors. "There's a crater gorged in the earth, the plane is pretty much disintegrated. There's nothing left but scorched trees," said one local, Mark Stahl.

There was immediate speculation that the plane had been heading for another high-profile target: Camp David, the US presidential retreat, which lies in the Maryland mountains 85 miles south-east of the crash site.

By mid-morning, the wide and normally crowded bridges across the Potomac were deserted and the scene resembled a city at war: deserted streets, billowing smoke and warplanes circling above. An elderly man, Tom O'Riordan standing in the shade of a tree near the Jefferson Memorial said he had not seen anything like it since Pearl Harbour.

A mobile secret service command center raced west on H Street, with sirens blaring, shortly after 11am as police drew a growing perimeter around the White House. Metal gates and yellow tape blocked access to streets and alleys. People scrambled to find working pay phones or reach friends or family on cell phones.

At 11.30, police cars again screamed up and down the roads around the Pentagon ordering passers-by off the street. One officer said there had been another report of an incoming plane heading down the Potomac river at high speed.

By midday, local hospitals reported receiving 40 victims of the attack, with seven patients in critical condition admitted to one facility for treatment of burns. Long lines of blood donors queued up outside area hospitals. Cardinal Theodore McCarrick, the city's Roman Catholic leader, said an unusually large number of worshippers, between 3,000 and 4,000, attended Mass at the downtown cathedral as the enormity of the destruction began to sink in.

By then, America had virtually ground to a halt. Almost every aspect of life, from sports occasions to family events, had been put on hold as the nation struggled to come to terms with what had happened. For several hours, the volume of people using the telephone service had made it impossible for anyone outside the US to phone in, and with international flights diverted away from the country, it had closed itself off to the outside world.

It was mid-afternoon before details of the hijacked planes started to emerge. The first announcement came from American Airlines, which confirmed that it had lost flight 11 from Boston to LA with 92 passengers and crew and flight 77 from Washington Dulles to LA with 64 people on board. Shortly after, United announced that the plane which had crashed in Pennsylvania was its flight 93, a Boeing 757 which had been en route to San Francisco from Newark, New Jersey. It had also lost another plane, flight 175, a Boeing 767 from Boston to LA.

Dismay Across the US, passengers queuing for flights and relatives waiting to meet arriving planes stood in airport lobbies staring at the arrival and departure monitors and listening with a growing sense of bewilderment and dismay to the announcements over the loudspeakers. Every major airport has had its rehearsals for disaster but not since Pearl Harbour had the country experienced such a widespread series of attacks.

Los Angeles International airport, the destination for three of the four hijacked flights, announced a suspension of operations as soon as it became clear what had happened. Worried callers were diverted to the lines of American Airlines and United, which were trying to supply information of who had been on the flights.

The airport itself was closed to the public and its operations suspended with only key staff allowed to remain. California governor Gray Davis made the National Guard available to assist.

Grief counsellors were called in by American Airlines and United to be ready to meet the friends and relatives of those on the flights. Switchboards were jammed as people tried to get information from the airport.

Lieutenant Howard Whitehead of the Los Angeles police said: "We are working with all the other agencies and a total evacuation of the airport has been ordered for precautionary reasons. Right now everything is fluid."